Because of the nature of conventional metal casting methods and molds, molten metal, when solidifying after casting, can experience variability in cooling rates. This has been recognized to result in a variety of casting discontinuities including porosity, microfissures and internal tears. Many of such discontinuities are well within the casting and unconnected with the casting surface. However, some are surface-connected through surface openings.
Hot isostatic pressing (HIP) processing, a production process, subjects a casting to a preselected combination of temperature and pressure to heal discontinuities, through the creeping together and diffusion bonding of the surfaces of internal porosity, microfissures, etc., or the homogenization of undesirable internal phase regions within the casting. In order to prevent pressurizing fluid used in such HIP processing from penetrating into surface openings of surface-connected discontinuities, coatings have been applied to the casting outer surface to bridge such openings. The HIP process and the application of such coatings is described in U.S. Pat. No. 3,758,347 - Stalker, issued Sept. 11, 1973, the disclosure of which is incorporated herein by reference. In that disclosure, application of the HIP process has been described in connection with alloys based on such elements as Ni, Co, Fe and Ti, although it should be recognized that the application of such process to other materials can be and have been made. For example, such application is contemplated for the closure of sub-surface porosity in aluminum or aluminum alloy castings in U.S. Pat. No. 3,496,624 - Kerr et al, issued Feb. 24, 1970, the disclosure of which is incorporated herein by reference. Thus, the HIP process can be practiced within a relatively broad temperature range, for example about 1,300-2,200.degree. F., depending upon the alloy being treated. The temperature is selected so that there will be substantially no degradation in the mechanical properties of the metal as a result of the HIP processing but sufficiently high in level to achieve diffusion bonding of the surfaces of the discontinuities. For example, in the range of about 1-30 thousand psi, sufficient pressure is applied to exceed the creep strength of the alloy being processed at the preselected temperature.
As is shown in the above-incorporated Stalker patent, surface-connected porosity can be healed through the use of a surface coating which prevents HIP pressurizing fluid penetration beneath the surface and into the surface-connected discontinuities. One type of coating which has been used for this purpose is a metal coating deposited over the surface-connected openings. Such a metallic coating in the form of nickel electro-plate is discussed in Example 2 of the above-incorporated Stalker patent. Further evaluation of the use of such metallic coatings has recognized that a change occurs in surface chemistry of the casting. Removal of such a changed surface can add undesirable cost to the process. Therefore, the present invention is an improvement on the invention of such incorporated Stalker patent through the provision of an improved coating useful in the above-described HIP process when applied to castings with surface-connected discontinuities.